1. Field of the Invention
The present invention relates to a device for controlling the air-fuel (A/F) ratio of an internal combustion engine, more particularly, to an A/F ratio controller which constantly maintains voltage to be supplied to the heater for heating the A/F ratio sensor at the predetermined value.
2. Description of the Prior Art
When operating an internal combustion engine, in particular, which drives such a vehicle engine provided with ternary catalyzer for purifying exhaust gas, the A/F ratio of exhaust gas must strictly be held at the theoretical A/F ratio. Today, there is such a specific A/F ratio controller offered for use, which executes feedback control of A/F ratio by means of an A/F ratio sensor which sharply varies the level of output by applying the theoretical A/F ratio in order that the actual A/F ratio can approximate the theoretical A/F ratio.
Nevertheless, since the A/F ratio sensor of the abovecited A/F ratio controller can merely measure the theoretical A/F ratio, actually, this controller cannot execute feedback control of A/F ratio covering an extensive range. To compensate for such disadvantage, recently, a preceding art presents a system for controlling the A/F ratio using an A/F ratio sensor which is capable of measuring not only the theoretical A/F ratio, but can also continuously measure the A/F ratio from the rich to the lean degree according to the volume of specific component like oxygen present in the exhaust gas. This A/F ratio sensor incorporates an oxygen-concentration detecting element composed of ion-conductive solid electrolyte and a heater which activates this element. Unless held at the predetermined temperature by means of the heater, the oxygen-concentration detecting element of this A/F ratio sensor cannot correctly function. FIG. 1 is the graphical chart designating the relationship between temperature of the oxygen-concentration detecting element and deviation of signals outputted from the above-cited A/F ratio sensor. As is clear from this chart, independent of differential degrees of temperature borne by the oxygen-concentration detecting element against the predetermined reference level, deviation is generated from signals outputted from the A/F ratio sensor.
Generally, heater is heated by power voltage outputted from battery. Variation of the power voltage causes the caloric value of the heater to become variable, and as a result, the oxygen-concentration detecting element cannot fully be heated to the predetermined temperature. This in turn causes the oxygen-concentration detecting element to malfunction itself, thus generating deviation in signals outputted from this element and eventually degrading accuracy in the feedback control of the A/F ratio.